The conventional surface treatment method of this kind, as shown in the Japanese Utility Model Publication No. 34025/1985 has boat shaped surface treatment baths arranged longitudinally along the direction in which the car bodies are carried. This method is a type of continuous transport in which the car bodies are successively immersed in one bath after another while being carried. This method has the advantage that the surface treatment bath is large, thus making the overall length of the surface treatment line long. Such a large bath requires a large amount of solution. Also, it is difficult to control the temperature and composition of the solution.
There is also known another type of surface treatment method which is called a batch type method in which carried car bodies are stopped and immersed in the baths. The Japanese Patent Publication No. 31536/1974 and the Japanese Utility Model Laid-Open No. 155374/1986 may be cited as belonging to this type of method. In the former case, a car body is held in a front-or rear-raised vertical posture in which the longitudinal direction of the car body is directed vertical when it is immersed in the bath. In the latter case, a car body is hung from a hanger of a hanger conveyor in such a manner that it is held horizontal in posture with its longitudinal axis extending along the processing line. The hanger, with the car body, is stopped at a predetermined position, where the car body is lowered or lifted by the hanger, to be immersed in or lifted out of the bath while remaining hung from the hanger.
While a batch type surface treatment method has an advantage that the bath can be made smaller and the problems experienced with the foregoing continuous transport type can thus be solved, the method disclosed in the Japanese Patent Publication No. 31536/1974 still has a problem. Since the car body is put into or taken out of the bath while held in the front- or rear-raised vertical posture, the bath has to be made deeper resulting in a significant increase in the overall height of the equipment as a whole including the car body hanging mechanism. Also, an increase in the time required to complete one cycle of operation is needed due to more time being needed for putting the car body into and taking it out of the bath.
The Japanese Utility Model Laid-Open No. 155374/1986, also has a drawback. Although it is free of the foregoing problems as the car body is immersed into or taken out of the bath in its horizontal posture, air may easily be trapped in the ceiling or recessed parts in the car body causing an uneven result of the treatment because the car body is held horizontal in a fixed position while immersed in the solution for the treatment. It is, therefore, desired that the surface treatment processing be carried out with small baths like those of this invention without producing an unevenly treated surface.